Catalyst for sulfite scavengers

ABSTRACT

ClO 2  and its water-soluble salts are catalysts for water-soluble bisulfites or sulfites when these latter compounds are used as oxygen scavengers in aqueous systems.

INTRODUCTION

Many industrial waters are treated to remove or reduce the oxygen contained therein. Illustrative of such waters are boiler waters and injection waters used in the secondary recovery of petroleum. For many years, it has been customary to treat oxygen-containing waters to remove or reduce the dissolved oxygen with inorganic bisulfites or sulfites such as sodium or ammonium bisulfite or sulfite. For purposes of simplification, the specification will hereafter refer to bisulfite(s) with it being understood that the sulfite(s) are included in such terminology. The bisulfite reaction with oxygen in these industrial applications requires 8 parts by volume of the water-soluble bisulfite to be added per parts by volume of molecular oxygen contained in the fluid being treated.

The bisulfite reaction with oxygen is slow. It is customary to use in conjunction with bisulfite oxygen scavengers small amounts of a catalyst to increase the rate of reaction between the dissolved oxygen and the bisulfite.

Various water-soluble cobalt-containing compounds are most commonly used as catalysts for promoting the bisulfite oxygen reaction. Cobalt catalyzed bisulfites have been used for many years as oxygen scavengers for treating boiler waters. When cobalt is used as a catalyst with soluble bisulfites to treat boiler waters which contain chelating agents or scale suppressants or are used to treat injection waters which are contaminated with H₂ S and are used as a flooding medium in the secondary recovery of petroleum, their effectiveness is greatly diminished. In the case of boiler water treatments, scale and corrosion inhibitors frequently complex with cobalt, rendering it unavailable as a catalyst. In the case of secondary recovery operations, these waters are frequently contaminated with H₂ S which forms an insoluble precipitate with cobalt which, again, nullifies its catalytic activity with respect to the bisulfite oxygen reaction.

It is now known that even cobalt catalyzed bisulfite oxygen reactions are measurably slowed down when the oxygen-containing waters contain substantial amounts of dissolved solids such as are found in natural brines or when the temperature of such waters is reduced to about the freezing point.

It would be a valuable contribution to the art if there were afforded an improved catalyst for the bisulfite oxygen reaction when such reaction is used to scavenge oxygen from water. Such catalyst should be capable of operating at low dosages. Further, it should be inexpensive, be water-soluble, and be immune from the effects of the water temperature, dissolved solids, and would not be affected by the presence of chemical scale inhibitors, chelating agents or hydrogen sulfide.

THE INVENTION

In accordance with the invention, it has been found that water-soluble bisulfite oxygen scavengers can be improved in their operational efficiency when they are used in the presence of a catalytic amount of a compound comprising chlorine dioxide or its water-soluble salts. The water-soluble salts of chlorine dioxide include the alkali metal or ammonium chlorites.

The amount of chlorine dioxide or water-soluble chlorite used in the practice of the invention may vary over a fairly broad range. As a general rule, it is possible to use as little as 0.001% to 50% by weight based on the bisulfite compound used as the oxygen scavenger. In most cases, quantities ranging from 0.01 to 25% by weight give good results with amounts ranging between 1-10% by weight giving excellent results in most cases. One of the findings of the invention resides in the fact that as the amount of catalyst is increased, the rate of the reaction is also increased.

In most applications, the sodium chlorite in the form of an aqueous solution will be employed. It is understood, however, that chlorine dioxide can be used by bubbling it into the aqueous solution to be treated or it can be bled into a causticized solution to produce sodium chlorite. Alternatively, if it is desired to use ClO₂, it may be generated by acidizing aqueous solutions of alkali metal chlorites.

In boiler water applications, the most commonly used water-soluble bisulfite scavenger is sodium bisulfite. The catalyst of the invention work extremely well with this material when it is used to scavenge oxygen from boiler waters.

In the case of the water flood applications, an extremely useful oxygen scavenger is ammonium bisulfite which is especially prepared for such applications. Illustrative of a typical ammonium bisulfite composition is an aqueous solution which contains between 60-65% of ammonium bisulfite. This material is prepared by adding anhydrous ammonia and sulfur dioxide to water while keeping the temperature below 75° C. A typical product of this type hereinafter listed as Composition 1:

    ______________________________________                                         Composition 1                                                                  ______________________________________                                         Material          Weight Percentage                                            ______________________________________                                         Water             23.80                                                        Anhydrous Ammonia 36.42                                                        Sulfur Dioxide    39.78                                                        ______________________________________                                                           100.00                                                       ______________________________________                                    

As previously indicated, sodium chlorite is a preferred catalyst for utilization in the practice of this invention. From a commercial standpoint, it is desirable that this material be made available in the form of an aqueous concentrate to facilitate its shipping to a use site. When concentrated solutions of sodium chlorite are prepared, there is a tendency for such solutions to decompose under conditions of long-term storage. To overcome this deficiency, we have devised a stable aqueous solution of sodium chlorite which contains relatively large amounts of sodium chlorite, is relatively non-viscous, and has a good "shelf" or storage life. This composition is set forth below as Composition 2:

    ______________________________________                                         Composition 2                                                                  ______________________________________                                         Ingredients      Weight Percentage                                             ______________________________________                                         Tap Water        74.0                                                          Sodium Carbonate 5.0                                                           Sodium Chloride  5.0                                                           Sodium Chlorite  16.0                                                          ______________________________________                                    

In the above formula, the sodium chloride portion of the formula tends to act as a pour point depressant whereas the sodium carbonate acts as a stabilizer to prevent decomposition of the sodium chlorite.

A generic concentrate of the type shown in Composition 2 is set forth in Composition 2A below:

    ______________________________________                                         Composition 2A                                                                 ______________________________________                                         Ingredients   Weight Percentage Range                                          ______________________________________                                         Tap Water     37.0 - 74.0                                                      Sodium Carbonate                                                                             2.5 - 5.0                                                        Sodium Chloride                                                                              2.5 - 5.0                                                        Sodium Chlorite                                                                              8.0 - 16.0                                                       ______________________________________                                    

EXAMPLES

To illustrate the invention, a variety of tests were run to determine the effectiveness of the chlorite catalyzed scavengers under a variety of conditions:

EXAMPLE 1 Sour Lab Tap Water

Results using Sugar Land, Texas, tap water containing 5-10 ppm H₂ S are shown on FIGS. I and II. These test results show that Composition 2 is an effective catalyst for Composition 1.

EXAMPLE 2 Correlate Tap Water

Results using tap water from the correlate system are shown on FIG. III. This water contained only a trace amount of iron sulfide (<1 ppm) and had a pH of 7.9. This test shows Composition 1 to be ineffective after 3 minutes, but with the addition of Composition 2, the oxygen is quickly scavenged.

EXAMPLE 3 Cold Sea Water

The effectiveness of Composition 2 as a catalyst for Composition 1 in cold sea water (32° F.) is shown on FIG. IV. The results show that without any catalyst, Composition 1 scavenges the oxygen at a very slow rate, but with the addition of 2.5 ppm of Composition 2, the oxygen is removed in 2 minutes.

EXAMPLE 4 Tap Water with Chelant or Scale Inhibitor

Results showing the effect of a chelant, ethylene-diamine tetraacetic acid or scale inhibitor (30% aqueous solution of triethanolamine phosphate) on Composition 1 in tap water are shown on FIGS. V and VI. Composition 2 makes Composition 1 very effective in the presence of a chelant or scale inhibitor.

EXAMPLE 5 Stability

The stability of Composition 1 diluted with water to produce 12% as ClO₂ is shown on FIG. VII. The neat chemical was placed in a 160° F. bath for 20 days and then checked for effectiveness as a catalyst for Composition 1 in sour tap water. The results show that Composition 1 diluted with water to produce 12% as ClO₂ had no loss of effectiveness after this time. 

Having thus described our invention, we claim:
 1. A method of scavenging oxygen from aqueous fluids of the type wherein a water soluble bisulfite or sulfite is added to such waters as the scavenger, the improvement which comprises using as a catalyst for the water soluble bisulfite or sulfite a compound from the group consisting of ClO₂ and its water-soluble salts.
 2. The method of claim 1 where the water-soluble bisulfite or sulfite is ammonium bisulfite or sulfite and the catalyst is sodium chlorite with the amount of sodium chlorite based on ammonium bisulfite or sulfite being within the range of 0.001 to 50% by weight.
 3. The method of claim 2 where the aqueous fluid is used as an injection fluid in the secondary recovery of petroleum. 